OAuth Working Group T. Lodderstedt, Ed.
Internet-Draft Deutsche Telekom AG
Intended status: Standards Track S. Dronia
Expires: November 20, 2013 M. Scurtescu
Google
May 19, 2013
OAuth 2.0 Token Revocationdraft-ietf-oauth-revocation-09
Abstract
This document proposes an additional endpoint for OAuth authorization
servers, which allows clients to notify the authorization server that
a previously obtained refresh or access token is no longer needed.
This allows the authorization server to cleanup security credentials.
A revocation request will invalidate the actual token and, if
applicable, other tokens based on the same authorization grant.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 20, 2013.
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From an end-user's perspective, OAuth is often used to log into a
certain site or application. This revocation mechanism allows a
client to invalidate its tokens if the end-user logs out, changes
identity, or uninstalls the respective application. Notifying the
authorization server that the token is no longer needed allows the
authorization server to clean up data associated with that token
(e.g. session data) and the underlying authorization grant. This
behavior prevents a situation where there is still a valid
authorization grant for a particular client which the end user is not
aware of. This way, token revocation prevents abuse of abandoned
tokens and facilitates a better end-user experience since invalidated
authorization grants will no longer turn up in a list of
authorization grants the authorization server might present to the
end-user.
2. Token Revocation
Implementations MUST support the revocation of refresh tokens and
SHOULD support the revocation of access tokens (see Implementation
Note).
The client requests the revocation of a particular token by making an
HTTP POST request to the token revocation endpoint URL. This URL MAY
include a query component. The means to obtain the location of the
revocation endpoint is out of scope of this specification. For
example, the client developer may consult the server's documentation
or automatic discovery may be used. As this endpoint is handling
security credentials, the endpoint location needs to be obtained from
a trustworthy source.
Since requests to the token revocation endpoint result in the
transmission of plain text credentials in the HTTP request, the
authorization server MUST require the use of a transport-layer
security mechanism when sending requests to the token revocation
endpoints. The authorization server MUST support TLS 1.0
([RFC2246]), SHOULD support TLS 1.2 ([RFC5246]) and its future
replacements, and MAY support additional transport-layer mechanisms
meeting its security requirements.
2.1. Revocation Request
The client constructs the request by including the following
parameters using the "application/x-www-form-urlencoded" format in
the HTTP request entity-body:
token REQUIRED. The token that the client wants to get revoked.
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token_type_hint OPTIONAL. A hint about the type of the token
submitted for revocation. Clients MAY pass this parameter in
order to help the authorization server to optimize the token
lookup. If the server is unable to locate the token using
the given hint, it MUST extend its search accross all of its
supported token types. An authorization server MAY ignore
this parameter, particularly if it is able to detect the
token type automatically. This specification defines two
such values:
* access_token: An Access Token as defined in [RFC6749]
section 1.4
* refresh_token: A Refresh Token as defined in [RFC6749]
section 1.5
Specific implementations, profiles, and extensions of this
specification MAY define other values for this parameter
parameter using the registry defined in Section 4.1.2.
The client also includes its authentication credentials as described
in Section 2.3. of [RFC6749].
For example, a client may request the revocation of a refresh token
with the following request:
POST /revoke HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
token=45ghiukldjahdnhzdauz&token_type_hint=refresh_token
The authorization server first validates the client credentials (in
case of a confidential client) and then verifies whether the token
was issued to the client making the revocation request. If this
validation fails, the request is refused and the client is informed
of the error by the authorization server as described below.
In the next step, the authorization server invalidates the token.
The client MUST assume the revocation is immediate upon the receipt
of an HTTP 200 response from the server. The client MUST NOT use the
token again after the revocation.
Depending on the authorization server's revocation policy, the
revocation of a particular token may cause the revocation of related
tokens and the underlying authorization grant. If the particular
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token is a refresh token and the authorization server supports the
revocation of access tokens, then the authorization server SHOULD
also invalidate all access tokens based on the same authorization
grant (see Implementation Note). If the token passed to the request
is an access token, the server MAY decide to revoke the respective
refresh token as well.
Note: A client compliant with [RFC6749] MUST be prepared to handle
unexpected token invalidation at any time. Independent of the
revocation mechanism specified in this document, resource owners may
decide to revoke authorization grants or the authorization server may
invalidate tokens in order to mitigate security threats. Thus having
different server policies with respect to cascading the revocation of
tokens should not pose interoperability problems.
2.2. Revocation Response
The authorization server responds with HTTP status code 200 if the
token has been revoked sucessfully or if the client submitted an
invalid token. The content of the response body does not matter as
all information is conveyed in the response code.
An invalid token type hint value is ignored by the authorization
server and does not influence the revocation response.
2.2.1. Error Response
The error presentation conforms to the defintion in section 5.2 of
[RFC6749]. The following additional error code is defined for the
token revocation endpoint:
unsupported_token_type The authorization server does not support the
revocation of the presented token type. I.e. the client
tried to revoke an access token on a server not supporting
this feature.
If the server responds with HTTP status code 503, the client must
assume the token still exists and may retry after a reasonable delay.
The server may include a "Retry-After" header in the response to
indicate how long the service is expected to be unavailable to the
requesting client.
2.3. Cross-Origin Support
The revocation end-point MAY support CORS [W3C.WD-cors-20120403] if
it is aimed at use in combination with user-agent-based applications.
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In addition, for interoperability with legacy user-agents, it MAY
also offer JSONP [jsonp] by allowing GET requests with an additional
parameter:
callback OPTIONAL. The qualified name of a JavaScript function.
For example, a client may request the revocation of an access token
with the following request (line breaks are for display purposes
only):
https://example.com/revoke?token=agabcdefddddafdd&
callback=package.myCallback
Successful response:
package.myCallback();
Error response:
package.myCallback({"error":"unsupported_token_type"});
Clients should be aware that when relying on JSONP, a malicious
revocation end-point may attempt to inject malicious code into the
client.
3. Implementation Note
OAuth 2.0 allows deployment flexibility with respect to the style of
access tokens. The access tokens may be self-contained so that an
resource server needs no further interaction with an authorization
server issuing these tokens to perform an authorization decision of
the client requesting access to a protected resource. A system
design may, however, instead use access tokens that are handles
referring to authorization data stored at the authorization server.
This consequently requires a resource server to issue a request to
the respective authorization server to retrieve the content of the
access token every time a client presents an access token.
While these are not the only options they illustrate the implications
for revocation. In the latter case the authorization server is able
to revoke an access token previously issued to a client when the
resource server relays a received access token. In the former case
some (currently non-standardized) backend interaction between the
authorization server and the resource server may be used when
immediate access token revocation is desired. Another design
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alternative is to issue short-lived access tokens, which can be
refreshed at any time using the corresponding refresh tokens. This
allows the authorization server to impose a limit on the time revoked
access tokens are in use.
Which approach of token revocation is chosen will depend on the
overall system design and on the application service provider's risk
analysis. The cost of revocation in terms of required state and
communication overhead is ultimately the result of the desired
security properties.
4. IANA Considerations4.1. OAuth Extensions Error Registration
This specification registers the following error values in the OAuth
Extensions Error registry defined in [RFC6749].
4.1.1. The "unsupported_token_type" Error Value
Error name unsupported_token_type
Error usage location revocation endpoint error response
Related protocol extension Token Revocation Endpoint
Change controller IETF
Specification document(s) [this document]
4.1.2. OAuth Token Type Hint Registry
This specification establishes the OAuth Token Type Hint registry.
Possible values of the parameter "token_type_hint" (see Section 2.1)
are registered with a Specification Required ([RFC5226]) after a two-
week review period on the TBD@ietf.org mailing list, on the advice of
one or more Designated Experts. However, to allow for the allocation
of values prior to publication, the Designated Expert(s) may approve
registration once they are satisfied that such a specification will
be published. Registration requests must be sent to the TBD@ietf.org
mailing list for review and comment, with an appropriate subject
(e.g., "Request for parameter: example"). Within the review period,
the Designated Expert(s) will either approve or deny the registration
request, communicating this decision to the review list and IANA.
Denials should include an explanation and, if applicable, suggestions
as to how to make the request successful. IANA must only accept
registry updates from the Designated Expert(s) and should direct all
requests for registration to the review mailing list.
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Hint Value: The additional value, which can be used to indicate a
certain token type to the authorization server.
Change controller: For Standards Track RFCs, state "IETF". For
others, give the name of the responsible party. Other details
(e.g., postal address, email address, home page URI) may also be
included.
Specification document(s): Reference to the document(s) that specify
the type, preferably including a URI that can be used to retrieve
a copy of the document(s). An indication of the relevant sections
may also be included but is not required.
4.1.2.2. Initial Registry Contents
The OAuth Token Type Hint registry's initial contents are:
o Hint Value: access_token
o Change controller: IETF
o Specification document(s): [this document]
o Hint Value: refresh_token
o Change controller: IETF
o Specification document(s): [this document]
5. Security Considerations
If the authorization server does not support access token revocation,
access tokens will not be immediately invalidated when the
corresponding refresh token is revoked. Deployments must take this
into account when conducting their security risk analysis.
Cleaning up tokens using revocation contributes to overall security
and privacy since it reduces the likelihood for abuse of abandoned
tokens. This specification in general does not intend to provide
countermeasures against token theft and abuse. For a discussion of
respective threats and countermeasures, consult the security
considerations given in section 10 of the OAuth core specification
[RFC6749] and the OAuth threat model document [RFC6819].
Malicious clients could attempt to use the new endpoint to launch
denial of service attacks on the authorization server. Appropriate
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countermeasures, which should be in place for the token endpoint as
well, MUST be applied to the revocation endpoint (see [RFC6819],
section 4.4.1.11). Specifically, invalid token type hints may
misguide the authorization server and cause additional database
lookups. Care MUST be taken to prevent malicious clients from
exploiting this feature to launch denial of service attacks.
A malicious client may attempt to guess valid tokens on this endpoint
by making revocation requests against potential token strings.
According to this specification, a client's request must contain a
valid client_id, in the case of a public client, or valid client
credentials, in the case of a confidential client. The token being
revoked must also belong to the requesting client. If an attacker is
able to successfully guess a public client's client_id and one of
their tokens, or a private client's credentials and one of their
tokens, they could do much worse damage by using the token elsewhere
than by revoking it. If they chose to revoke the token, the
legitimate client will lose its authorization grant and will need to
prompt the user again. No further damage is done and the guessed
token is now worthless.
Since the revocation endpoint is handling security credentials,
clients need to obtain its location from a trustworthy source only.
Otherwise, an attacker could capture valid security tokens by
utilizing a counterfeit revocation endpoint. Moreover in order to
detect counterfeit revocation endpoints, clients MUST authenticate
the revocation endpoint (certificate validation etc.).
6. Acknowledgements
We would like to thank Peter Mauritius, Amanda Anganes, Mark Wubben,
Hannes Tschofenig, Michiel de Jong, Doug Foiles, Paul Madsen, George
Fletcher, Sebastian Ebling, Christian Stuebner, Brian Campbell, Igor
Faynberg, Lukas Rosenstock, and Justin Richer for their valuable
feedback.
7. References7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
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